Porphyromonas gingivalis is an important gram-negative periopathogen strongly associated with adult type periodontitis. P. gingivalis lipopolysaccharide (LPS) displays an unusual amount of lipid A structural heterogeneity which we hypothesized may be a potential modulator of the innate host defense response. During the previous funding period we discovered a novel molecular mechanism used by P. gingivalis to evade and subvert the TLR4 component of human innate immune system. Signal transduction following binding of lipopolysaccharide (LPS) to Toll-like receptor 4 (TLR4) is an essential aspect of host innate immune responses to infection by Gram negative pathogens. We found that P. gingivalis, uses endogenous lipid A 1- and 4'-phosphatase activities to modify its LPS, creating immunologically silent, nonphosphorylated lipid A. This unique lipid A provides a highly effective mechanism employed by this bacterium to evade TLR4 sensing and to resist killing by cationic anti- microbial peptides. Therefore our overall hypothesis for this renewal application is: P. gingivalis modulates its interactions with the host innate defense system through regulation of lipid A phosphatase activity. Specifically we have found that hemin regulates the lipid A structural composition of P. gingivalis such that a low hemin concentration a TLR4 silent LPS is made whereas at high hemin concentrations a TLR4 antagonist lipid A is found. Our results indicate that the hemin concentration regulation of lipid A phosphatase activity shifts P. gingivalis lipid A activity from TLR4 evasive to TLR4 suppressive, potentially altering critical interactions between this bacterium, the local microbial community, and the host innate immune system. Our hypothesis will be examined by directly determining lipid A 1 and 4' phosphatase enzymatic activity (Aim 1), characterizing lipid A 1 and 4' phosphatase protein expression (Aim 2), and genetic regulation (Aim 3). Furthermore, Aim 4 will examine the ability of the TLR4 evasive and suppressive lipid A structures to alter the local microbial community associated with disease and the host innate immune system in a rabbit model of periodontitis. These studies will elucidate the mechanisms by which P. gingivalis regulates its lipopolysaccharide interactions with the innate host defense system and test the contribution of lipid A structural regulation in an animal model of disease.